Suddenly, Oklahoma is earthquake country

1of8SUE OGROCKI : ASSOCIATED PRESS HIT TWICE: Joe Reneau of Sparks, Okla., said he filled a trash can with items damaged in a Saturday morning quake. The other items spilled out during a quake Saturday night.Photo: Sue Ogrocki

2of8Oklahoma State safety Markelle Martin (10) breaks up a pass intended for Kansas State wide receiver Tyler Lockett, left, late in the fourth quarter of an NCAA college football game in Stillwater, Okla., Saturday, Nov. 5, 2011. Oklahoma State won 52-45. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

3of8Chad Devereaux works to clear up bricks that fell from three sides of his in-laws' home in Sparks, Okla., Sunday, Nov. 6, 2011, after two earthquakes hit the area in less than 24 hours. The weekend earthquakes were among the strongest yet in a state that has seen a dramatic, unexplained increase in seismic activity. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

4of8Jess Burrow, left, and James Patterson, look over the damage caused outside the home of Joe and Mary Reneau when their chimney was toppled by Saturday's earthquake, in Sparks, Okla., Sunday, Nov. 6, 2011. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

5of8The exterior chimney at the home of Joe and Mary Reneau is pictured in Sparks, Okla., Sunday, Nov. 6, 2011, after it collapsed in Saturday night's earthquake. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

6of8The mirror from a bathroom in the home of Joe and Mary Reneau lies in pieces on the floor in Sparks, Okla., Sunday, Nov. 6, 2011, following Saturday night's earthquake. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

7of8Maintenance workers inspect the damage to one of the spires on Benedictine Hall at St. Gregory's University in Shawnee, Okla., Sunday, Nov. 6, 2011. Two earthquakes in the area in less than 24 hours caused one of the towers to topple, and damaged the remaining three. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

8of8A cookie jar lies in pieces on the kitchen counter as Jesse Richards describes what the earthquake felt like in Sparks, Okla., Sunday, Nov. 6, 2011. Oklahoma residents more accustomed to tornadoes than earthquakes have been shaken by weekend temblors that cracked buildings, buckled a highway and rattled nerves. One quake late Saturday was the state's strongest ever and jolted a college football stadium 50 miles away. (AP Photo/Sue Ogrocki)Photo: Sue Ogrocki

Usually, the earth-moving events on an autumn weekend in Oklahoma are at the college football stadiums in Norman and Stillwater.

But on this Saturday and Sunday, the earth moved - at least 23 times - and football was not involved.

The quakes, powerful by Midwestern standards, shook towns about an hour's drive northeast of Oklahoma City. They began early Saturday and continued intermittently through the weekend.

Late Saturday night, the area experienced the strongest earthquake ever recorded in the state. No serious injuries were reported, but minor damage to roads and buildings was reported, according to the Sheriff's Department in Lincoln County, the epicenter for many of the quakes.

Shallow quake

Geological activity in the region has increased in recent years, and earthquakes have occurred with greater frequency and intensity. The big quake on Saturday night, which occurred at 10:53, had a preliminary magnitude of 5.6, according to the United States Geological Survey.

Saturday night's quake was shallow, about three miles deep, and the epicenter was four miles east of Sparks, which is about 44 miles northeast of Oklahoma City.

That quake followed smaller ones earlier in the day, including one at 2:12 a.m. with a magnitude of 4.7. Its epicenter was in Prague, about 50 miles east of Oklahoma City.

Since mid-2009, the state has had 10 times more earthquakes than normal, said Austin Holland, a research seismologist with the Oklahoma Geological Survey. In 2010, the earth beneath Oklahomans' feet moved more than 1,000 times, but only 100 or so were strong enough to be felt. "We have not a clue," Holland said of the increase. "It could be a natural cycle. We just don't know."

Faults buried deep

Unlike earthquake-prone California and Japan, Oklahoma does not rest atop the fractious areas where two tectonic plates rub against each other. But the state's geophysical activity has only been surveyed in earnest for about 50 years, Holland said, making it difficult to draw conclusions or put the recent activity into context.

But the state does have faults that are buried deep, like the Wizetta Fault, also known as the Seminole Uplift, east of Oklahoma City, where pressure can build.

"You still get earthquakes within the plate. That doesn't mean there's a plate boundary, but there's a fault," said Don Blakeman, a geophysicist at the National Earthquake Information Center.

Collapsed chimneys

That pressure is released as tremors like those that startled residents over the weekend in Prague, Okla., where the visible damage on Sunday morning included chimneys that had collapsed onto houses. Parts of Highway 62 between Prague and Meeker buckled, Holland said.

In Lincoln County, cracks ran up the brick courthouse in Chandler after a smaller quake early Saturday morning, said Justin Reese, who runs the Boomarang Diner there.

Since the Saturday night quake, there have been 11 aftershocks that measured above 2.5 on the Richter scale, Blakeman said.

Holland said the intensity of the Oklahoma earthquakes "could go either way."